The present invention relates generally to inertial measurement units. More particularly, the present invention relates to detecting stationarity of an inertial measurement unit.
An inertial measurement unit (IMU) is a self-contained system which can automatically determine the position, velocity and attitude of a moving vehicle or other body. Based on prior knowledge of initial position, initial velocity, and initial orientation relative to a known reference frame, an IMU is capable of determining its present position, velocity and orientation without the aid of external information. Typical IMUs operate by sensing the linear acceleration and deceleration of the body in three-dimensional space with accelerometers and by sensing the rotational velocity, or rotational rate, with gyroscopes. Based on the sensed linear acceleration, the IMU calculates changes in linear velocity and, in turn, changes in position. Based on the sensed rotational velocity, the IMU calculates changes in rotational orientation, or attitude. The navigational data generated by IMUs are commonly used to determine the future course for a body to follow in order to bring it to its intended destination. IMUs are commonly used in the guidance and control of aircraft, missiles, spacecraft, ships and submarines, as well as for personal navigation.
IMU-based navigational systems are not as accurate as some other navigational systems such as GPS (Global Positioning Satellite) systems. The IMU inevitably senses noise in the acceleration and velocity data. This causes the accuracy of the IMU to degrade over time until the actual velocity and position of the body is updated. Often IMUs are used to supplement more accurate systems such as GPS or are used as backup to such systems. Also, there are locations where GPS signals are not available. Thus, there exists a need for a more accurate IMU.
The present invention provides a solution to this and other problems and offers other advantages over the prior art.
The present invention relates to detecting stationarity of an inertial measurement unit.
One embodiment of the present invention is directed to a device for measuring the velocity and/or position of a body. The device includes a measurement unit and a magnetometer. The measurement unit is capable of sensing an acceleration and/or a velocity of the body. The measurement unit is further capable of determining a velocity value and/or a position value based on the sensed acceleration and/or the sensed velocity of the body. The magnetometer is capable of sensing the magnetic flux to which the body is exposed, thereby determining whether the body is at rest.
In one embodiment of such a device, if the sensed magnetic flux is substantially constant, the velocity value is set at zero and the position value is held at its present value.
Another embodiment of the present invention is directed to a method of reducing degradation of the accuracy of an inertial measurement unit. The magnetic flux to which the inertial measurement unit is exposed is sensed. Stationarity conditions are invoked if the sensed magnetic flux is substantially constant.
In one embodiment of such a method, invoking stationarity conditions involves setting the velocity value at zero and/or holding the position value constant at its present value.